Multi-antenna/multi-receiver array diversity system

ABSTRACT

The present invention is a system for increasing Signal-to-Noise Ratio (SNR) in a wireless communication system comprising a plurality of antennas each antenna providing a signal, a device for selecting a subset of signals provided by the plurality of antennas, a maximum ratio combiner for summing the selected subset of signals provided by the plurality of antennas, and a decision device for measuring the selected subset of signals against a predefined threshold. The device for selecting the subset of signals is coupled to the plurality of antennas. The maximum ratio combiner is coupled to the selected subset of signals and the decision device for measuring the selected subset of signals against a predefined threshold. The decision device is coupled to the selecting device such that one selected signal of the selected subset of signals is replaced by an unused signal provided by the plurality of antennas.

FIELD OF THE INVENTION

The invention relates to multiple-antenna and multiple-receiver arraysfor use in increasing Signal-to-Noise Ratio (SNR) at the receiver. Inparticular, the present invention uses polarization and spatialdiversity to improve the SNR at the receiver.

BACKGROUND OF THE INVENTION

In wired or fiber systems, there is primarily only one signal path whichprovides a clean, non-interfering channel through which the signalpropagates. In wireless communication systems, signal propagation ismainly by way of scattering from the surfaces of buildings, cars, trees,walls and furniture and by diffraction over and/or around these objects,causing the transmitted signal to arrive at the receiver via multiplepaths through the air. The collection of propagation paths traversed bythe signals from the transmitter to the receiver is called the channel.Due to the multipath effect, signals may arrive in-phase or out-of-phasewith one another and at varying amplitude levels. To further complicatematters, as the physical objects within the channel move (i.e. thereceiver, transmitter or objects in the path between the transmitter andreceiver), the channel changes. This provides a time-varying componentto almost all wireless channels. The effect of this time-varying channelis a time-varying received signal amplitude and phase at the receiver.Without using techniques to compensate for this variation, extrasignal-to-noise margin must be maintained at the receiver to ensurereliable communications.

Diversity is one technique used to combat time-varying channel effects.Diversity may be used in any combination within the time, frequency,polarization, or spatial domains. Simple diversity techniques canprovide tremendous improvements in the signal level at the receiver. Thekey, as in any technique, is to provide the maximum benefit for theminimum penalty (size, cost, etc.).

SUMMARY OF THE INVENTION

The multi-antenna/multi-receiver switch array diversity system of thepresent invention uses spatial and polarization diversity to improveperformance at the receiver. Four antennas at the receiver provide fourunique (uncorrelated or low-correlation) versions of the signal at theirterminals. Positions 1 and 2 (providing spatial diversity) each have twoantennas in cross-polarization (A&B) to one another. Thecross-polarization provides polarization diversity. Thus, the fourantennas are both polarization and spatially diverse. This providespolarization diversity for each spatially diverse position.

The present invention takes advantage of a blind switching scheme basedon a predefined threshold and algorithm to combine dually polarizedand/or multiple antennas to increase SNR at the receiver. The increasein the SNR may be used to decrease the Bit Error Rate (BER), increasethe overall Quality of Service (QoS) or reduce transmit powerrequirements for narrowband or wideband communication systems operatingin wireless environments.

The switched antenna array concept of the present invention seeks tooptimize the tradeoff between the benefit from diversity implementationand penalty in terms of size and cost to implement. The novelty of thepresent invention centers around the combined use of spatial andpolarization diversity.

It is, therefore, an object of the present invention to use polarizationand spatial diversity to increase SNR at the receiver.

It is another object of the present invention to use a blind switchingalgorithm to increase SNR at the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best described with reference to the detaileddescription and the following figures, where:

FIG. 1 is a block diagram of the switched array diversity system;

FIG. 2 is a state diagram of the switch diversity array concept;

FIG. 3 shows the probability of received signal power being less than anabscissa envelope power in dB;

FIG. 4 shows the improvement in path loss; and

FIG. 5 shows the plot of average received power at the output of thereceiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, four antennas at the receiver provide fourunique (uncorrelated) versions of the signal at their terminals.Positions 1 and 2 (providing spatial diversity) each have two antennasin cross-polarization to one another, thus providing polarizationdiversity. Having four antennas that are both polarization and spatiallydiverse provides polarization diversity for each spatially diverseposition.

One option at this point would be to maximally combine the signals atall four of the antennas into the receiver. Each signal would requireits own receiver chain, making this a costly option. In addition,simulations and field results show that one or two signals dominate thecontribution at the receiver. The two highest signals are chosen fromthe four available signals.

Preferably, an algorithm is used that combines any two of these into amaximal ratio combining receiver. The advantage of this approach istwofold:

-   1. Only one spatial distance is required to provide diversity    (typically one wavelength) between spatially diverse antennas at    position 1 and position 2. With the spatial separation between    positions 1 and 2, each antenna is unique (diverse) from the    remaining three and only one wavelength physical separation was    required to do this.-   2. By using the two highest signals, most of the benefit from the    four input case of maximal combination could be obtained for half    the receiver cost.

FIG. 1 is a block diagram of the switched antenna array diversity systemconcept. Four input signals P₁₁, P₁₂, P₂₁ and P₂₂ are present with P₁₁representing antenna position 1 with polarization A, P₁₂ representingantenna position 1 with polarization B, P₂₁ representing antennaposition 2 and polarization A and P₂₂ representing antenna position 2and polarization B. Switch-1 105 and switch-2 110 each choose from amongthe four signals. Each switch 105 and 110 selects the highest signal butthe switches are prevented from selecting the same signal and do notswitch at the same time. From the four input signals present, two (R₁and R₂) are chosen and maximally combined using a maximum ratio combiner120 to provide a composite signal. As shown, any two of the signals(represented by γ₁ and γ₂) may be combined with one another. Each of thesignals may be adjusted by a coefficient, e.g., α₁ and α₂. The twoselected signals (R₁ and R₂) are inspected in the “decision block” 115and if one of the signals drops below a predefined threshold, one of theunused signals is chosen and combined. The decision process for choosingthe best signal inputs is continual.

The present invention chooses the two largest or highest antenna inputsfrom a choice of four available input signals and combines them in amaximal-ratio combiner 120 to produce an output of higher value andgreater consistency than in two-input diversity systems. Additionally,signal γ₁ may be adjusted by coefficient α₁ and signal γ₂ may beadjusted by coefficient α₂. The algorithm of the present inventioncontinually inspects each input in the “decision block” 115 and comparesit to a pre-defined threshold value. Should the value at an input dropbelow the threshold value, the algorithm chooses another input.

FIG. 2 is a state diagram of the switch diversity array concept andprovides the algorithm for switching between states. Notice that onlyone antenna is switched at a given time; this ensures that no transientsare present at the output, which would interrupt the signalcommunication flow. While the algorithm is blind (no information aheadof time), it provides benefit since statistically there is only a smalllikelihood that two signals will be in a fade (at a low level) at thesame time.

The algorithm for deciding if one of the two selected input signals hasfallen below a pre-defined threshold, thereby requiring selection ofanother input signal, is as follows. The antennas are ranked with therank being defined by antenna number. Antenna numbers can be assigned inany reasonable manner. When the metric on one of the antennas fallsbelow the pre-defined threshold, a switch is made to the availableantenna of the next highest rank. The leftmost column indicates switchstate. The next column to the right indicates the possible antenna paircombinations. If the highest rank antenna is to be switched, then itsrank is re-assigned the rank of 0 before the algorithm is applied. Theavailable antenna set is defined as the set {(P₁₁, P₁₂), (P₁₁, P₂₁),(P₁₁, P₂₂), (P₁₂, P₂₁), (P₁₂, P₂₂), (P₁₂, P₁₁), (P₂₁, P₁₁), (P₂₁, P₁₂),(P₂₁, P₂₂), (P₂₂, P₁₁), (P₂₂, P₁₂), (P₂₂, P₂₁)}—(pair of antennascurrently being used)}. Referring again to FIG. 2, if antenna pair (P₁₁,P₁₂) is currently being used and it is found that the signal at P₁₂ hasfallen below the pre-defined threshold, then a switch is made from P₁₂to P₂₁ so that the antenna pair to be used becomes (P₁₁, P₂₁) asindicated by the dashed line going from the antenna pair (P₁₁, P₁₂) toantenna pair (P₁₁, P₂₁). If, once again using antenna pair (P₁₁, P₁₂),it is found that P₁₁ has fallen below the pre-defined threshold, then aswitch should be made from P₁₁ to P₁₂ but P₁₂ is already in use as oneof the input signals of the currently used pair. The next highestavailable rank antenna would have to be selected and that would be P₂₁so that the antenna pair to be used becomes (P₂₁, P₁₂) as indicated bythe dashed and dotted line going from antenna pair (P₁₁, P₁₂) to antennapair (P₂₁, P₁₂). The plurality of states indicates time progression andthat the process of antenna pair selection is continual over the life ofthe signal transmissions. The dashed and dotted lines on FIG. 2 thusrepresent state movement from an antenna pair when the higher antenna ofthe pair in the current antenna pair set has fallen below thepre-defined threshold and needs to be switched. The dashed lines on FIG.2 thus represent state movement from an antenna pair when the lowerantenna of the pair in the current antenna pair set has fallen below thepre-defined threshold and needs to be switched.

In FIG. 3 the received signal envelope has a Rayleigh distribution. FIG.3 shows the probability of received signal power being less than anabscissa envelope power in dB. FIG. 4 shows the path loss. FIGS. 3 and 4provide simulation results for the statistical path gain improvementthat arises from the use of the present invention. Inspection of thesefigures shows the improvement obtained using the present invention. Inan ideal channel (such as cable or fiber), the plot in FIGS. 3 and 4would be a vertical line in each case (on each figure) because theprobability of receiving the signal is the same (or no variation in thereceived signal due to multipath). With the use of the presentinvention, the plot sharpens and becomes much more vertical, varyingabout 15 dB (indicating that the invention actually improves not onlythe median received power but it also improves the fade marginrequirements for a given BER) range as indicated on FIG. 4.

Specifically, referring to FIG. 3, the maximal ratio combination of eachof the possible antenna combinations is plotted. That is, antenna 1 withpolarization A (ant 1, p1), antenna 1 with polarization B (ant 1, p2),antenna 2 with polarization A (ant 2, p1) and antenna 2 withpolarization B (ant 2, p2) are plotted. Also plotted are the use of ablind switch, a four-receiver system that combines any two signals andthen selects the maximum, and a four-receiver system that uses an equalgain combination. FIG. 4 plots the same combinations.

FIG. 5 shows the plot of average received power at the output of thereceiver. Inspection of the figure shows that, for the two-inputmaximally combined case, the received power remains relatively constantover the time period. This is the topmost line in the graph.

It should be clear from the foregoing that the objectives of theinvention have been met. While particular embodiments of the presentinvention have been described and illustrated, it should be noted thatthe invention is not limited thereto since modifications may be made bypersons skilled in the art. The present application contemplates any andall modifications within the spirit and scope of the underlyinginvention disclosed and claimed herein.

1. A system for increasing Signal-to-Noise Ratio (SNR) in a wirelesscommunication system, comprising: a plurality of antennas, each antennaproviding a signal; a device, coupled to said plurality of antennas, forselecting a subset of signals provided by said plurality of antennas; amaximal-ratio combiner responsive to said subset of signals fordeveloping an output signal; and a decision device for measuring theeach signal of said subset against a predefined threshold, and forreplacing a signal z of said subset of signals with a signal of saidplurality of antennas that is not currently selected when said measuringdetermines that said signal z ought to be replaced.
 2. The systemaccording to claim 1, wherein said plurality of antennas furthercomprises four antennas.
 3. The system according to claim 2, whereinsaid four antennas are arranged such that there are two antennas at eachof two positions to accomplish spatial diversity and each of said twoantennas at each of said two positions are in cross-polarization witheach other in order to accomplish polarization diversity.
 4. The systemaccording to claim 1, wherein said selecting device selects the twolargest signals from said signals provided by said plurality ofantennas.
 5. A method for increasing Signal-to-Noise Ratio (SNR) in awireless communication system, comprising the steps of: receiving asignal from each of a plurality of antennas; selecting a subset ofsignals provided by said plurality of antennas; summing said selectedsubset of signals provided by said plurality of antennas; measuring eachsignal of said selected subset of signals against a predefinedthreshold; and periodically determining, based on said step ofmeasuring, whether one of the signals of said selected subset ought tobe replace, and replacing said one signal with another of said selectedsubset of signals provided by said plurality of antennas when saiddetermining concludes that said one of the signals has fallen below saidthreshold.
 6. The method according to claim 5, further comprising thestep of optionally adjusting each of said selected subset of signalsprovided by said plurality of antennas by a coefficient.
 7. The methodaccording to claim 5, wherein said plurality of antennas furthercomprises four antennas.
 8. The method according to claim 7, whereinsaid four antennas are arranged such that there are two antennas at eachof two positions to accomplish spatial diversity and each of said twoantennas at each of said two positions are in cross-polarization witheach other in order to accomplish polarization diversity.
 9. The methodaccording to claim 5, wherein said selecting step selects the twolargest signals from said signals provided by said plurality ofantennas.
 10. A system for increasing Signal-to-Noise Ratio (SNR) in awire less communication system, comprising the steps of: means forreceiving a signal from each of a plurality of antennas; means forselecting a subset of signals provided by said plurality of antennas;means for summing said selected subset of signals provided by saidplurality of antennas; means for measuring each signal of said selectedsubset of signals against a predefined threshold; and means forreplacing one signal of said selected subset of signals provided by saidplurality of antennas that has fallen below said threshold in a singletime cycle.
 11. The system according to claim 10, further comprisingmeans for optionally adjusting each of said selected subset of signalsprovided by said plurality of antennas by a coefficient.
 12. The systemaccording to claim 10, wherein said plurality of antennas furthercomprises four antennas.
 13. The system according to claim 12, whereinsaid four antennas are arranged such that there are two antennas at eachof two positions to accomplish spatial diversity and each of said twoantennas at each of said two positions are in cross-polarization witheach other in order to accomplish polarization diversity.
 14. The systemaccording to claim 10, wherein said means for selecting selects the twolargest signals from said signals provided by said plurality ofantennas.